The present invention relates to a thin film semiconductor device and a method of fabricating the semiconductor device, and more particularly to a thin film transistor whose characteristics can be improved effectively by hydrogenation and a method of fabricating the transistor.
Liquid crystal display devices are now widely used, since they are small in weight, thickness and power consumption. Especially, attention is paid to a liquid crystal plane display device, in which a thin film transistor (hereinafter simply referred to as "TFT") is connected to each pixel, to make possible the active matrix addressing. The performance of this display device is influenced by the characteristics of the TFT. The characteristics of TFT can be greatly improved by the hydrogenation of polycrystalline silicon which is used for making the TFT. For example, a method of diffusing hydrogen into polycrystalline silicon through an SiN film which is formed by the plasma CVD method, to hydrogenate the polycrystalline silicon, is discussed in an article (IEEE, Electron Device Lett., Vol. EDL-5, Nov. 11, 1984, pages 468 to 470). As described in the above article, metals such as molybdenum, molybdenum silicide and tungsten are impenetrable to hydrogen but polycrystalline silicon (hereinafter simply referred to as "poly-Si") is penetrable to hydrogen.
However, it has been found by the inventors' experiments that hydrogen can penetrate into only a surface layer of poly-Si having a limited thickness. Accordingly, in a case where a TFT does not have a favorable structure, it is impossible to hydrogenate the channel layer of the TFT sufficiently. This problem of a conventional TFT which has a coplanar structure as shown in FIG. 5, will be explained below in detail, with reference to FIGS. 6 and 7. It is to be noted that the channel-length dependence of threshold voltage shown in FIG. 6 and the channel-length dependence of field effect mobility shown in FIG. 7 are both found by the present inventors. Referring to FIG. 5, an undoped poly-Si layer 10 is formed on an insulating substrate 1, and an n.sup.+ -source region 2 and an n.sup.+ -drain region 3 are formed in the poly-Si layer 10 through ion implantation techniques. A gate insulating film 5 is formed on a channel region 4 which is not implanted with any ion, and a gate electrode 6 made of poly-Si is provided on the gate insulating film 5. Further, a phospho-silicate glass film 7 serving as a protective film, an aluminum electrode 20 serving as a source electrode, and another aluminum electrode 30 serving as a drain electrode are formed. In many TFT's having the above structure, the channel-length dependence of threshold voltage V.sub.TH and the channel-length dependence of film effect mobility .mu..sub.FE were investigated before and after plasma hydrogenation. FIGS. 6 and 7 show the results of investigation. In each of FIGS. 6 and 7, a broken line indicates the threshold voltage or field effect mobility measured before the plasma hydrogenation, and a solid line indicates the threshold voltage or field effect mobility measured after the plasma hydrogenation. As is apparent from FIGS. 6 and 7, when the channel length L of TFT is made less than or equal to 10 .mu.m, the threshold voltage V.sub.TH is reduced and the field effect mobility .mu..sub.FE is increased by the hydrogenation. In other words, the hydrogenation is effective only for a TFT having a channel length L less than or equal to 10 .mu.m. The reason for this will be explained below, with reference to FIG. 5. As mentioned above, hydrogen can penetrate into only a surface portion of the gate electrode 6 made of poly-Si. Accordingly, hydrogen can penetrate into a limited portion of the channel region 4 through the phospho-silicate glass layer 7 interposed between gate electrode 6 and the aluminum electrode 20 or 30. Thus, only those portions of the channel region 4 which are indicated by a length .DELTA.L in FIG. 5, can be hydrogenated. Incidentally, the phospho-silicate glass film 7 and the gate insulating film 5 are both penetrable to hydrogen. Accordingly, the characteristics of TFT can be improved (that is, the threshold voltage V.sub.TH is reduced and the field effect mobility .mu..sub.FE is increased), as a ratio of the length .DELTA.L of the hydrogenated portion to the total length L of the channel region 4 is larger (that is, the length L of the channel region 4 is smaller). Unlike a TFT which is used as a two-dimensional element for LSI, a poly-Si TFT used in a display device has a channel length L of about 10 .mu.m or more in accordance with an OFF-resistance, a design rule and others. When various circuits are formed by using such TFT's, each of the circuits includes TFT's which are different in channel length L from each other. Accordingly, the characteristics of each circuit are affected by various values of channel length L. Thus, it is very difficult to actually design each circuit, and moreover it is impossible for each circuit to exhibit satisfactory characteristics.